SE-0236: Package Manager Platform Deployment Settings

[Aciid (Ankit Aggarwal)]

@owensd

it seems like the primary goals are:

1. Allow developers to specific a minimum platform version
2. Promote packages to be available for all platforms
3. Allow for new platforms to be added by default to all packages
4. Allow developers to specify that only certain platforms are supported

I think this nicely summarizes the high level goals of this feature!

@owensd

To me, this implies that platforms should only provide a mechanism for changing the default value for the minimum version.

So:

    platforms: [
       .macOS(.v10_13), .iOS(.v12)
    ]

Would mean that all platforms are supported, but macOS and iOS have version v10.13 and v12 as the deployment targets set.

This solves #1, #2, and #3 above.

Agreed. This seems like the most sensible default to me.

@owensd

Allowing developers to restrict platforms is a different goal that updating the minimum supported version. As such, I think it would be best to be a separate option (and frankly, outside of this particular proposal):

I also want to separate out that part into a different proposal but I don't agree that it should be a different option. I feel like it should be part of the main platforms API.

Maybe something like:

platforms: .platforms([.macOS(.v10_13), .iOS(.v12)], restricted: true)

restricted=true would mean: allow all listed platforms + any platform not known to the current tools version. This will solve #1, #3, #4 and enable restricting to a particular set of platforms while allowing bringing up new platforms.

With this approach, we don't require .unsupported and it will lead to a much cleaner API. Having to explicitly mark platforms as unsupported seems counter-intuitive and can lead to a massive list of platforms as support for Swift grows over time.